Conventionally in a coal-fired boiler, raw coal is introduced into a coal pulverizer (a mill) such as a vertical roller mill 10 illustrated in FIGS. 7, 8(a), and 8(b), and pulverized coal is used as fuel. In the vertical roller mill 10, a mill roller 13 rotates while turning on a mill table 12 placed on a lower side in a housing 11. Note that a reference numeral “14” in the figures denotes a coal feed tube through which raw coal is introduced.
Raw coal introduced into the vertical roller mill 10 is crushed between the mill table 12 and the mill roller, and is pulverized into pulverized coal. While being dried by hot air flowed out from a throat 15 arranged to surround the mill table 12, the pulverized coal is delivered to a fixed separator 20 disposed on an upper side in the housing 11 by an air flow. At this point, gravitational separation is performed, in which coarse particles having a large particle size drop down due to the force of gravity and return onto the mill table 12. Thus, the particles are repeatedly pulverized until the particle size thereof reaches a desired particle size.
After the above-described gravitational separation, i.e., primary separation, pulverized coal having product particles including coarse particles is further separated by the separator disposed above the mill table 12. Examples of the above-described separator include a fixed separator, a rotary separator, and a combination thereof. The separator illustrated in the figures is the fixed separator. Note that it has been known that the rotary separator is configured to perform separation using collision/inertia force generated by rotary vanes and exhibits a high separation performance.
The pulverized coal delivered by the flow of air is dried by hot air, and is separated while passing through the fixed separator 20. The separated pulverized coal passes through pulverized coal outlets 16 allowing communication between the inside of the fixed separator 20 and the outside of the housing 11 on the upper side thereof, and then, is delivered to a boiler by the flow of primary air for delivery.
The fixed separator 20 includes many fixed blade inlet windows 22 opening at regular intervals in a circumferential direction on an upper end side of a cone 21. The fixed blade inlet windows 22 are openings formed to penetrate a wall forming the cone 21, and serve as the inlets and flow paths through which the flow (hereinafter referred to as a “solid-gas two-phase flow”) of air for delivering pulverized coal flows into the cone 21.
Many fixed blades 23 each paired with a corresponding one of the fixed blade inlet windows 22 are attached to an inner wall side of the cone 21.
Moreover, an inner cylinder 24 forming a wall facing the fixed blade inlet windows 22 and the fixed blades 23 is provided in the cone 21.
All of the fixed blades 23 incline in the same direction in order to swirl the solid-gas two-phase flow. That is, each fixed blade 23 is attached to have an inclination angle θ with respect to a line along a radial direction toward the center of the cone 21. Thus, with an increase/decrease in the inclination angle θ of the fixed blade 23, the strength of the swirl flow also changes according to the opening degree (the angle) of the fixed blade 23. Thus, the fineness of coal targeted for separation can be adjusted.
Note that a reference numeral “25” in the figures denotes a cone outlet through which raw coal and coarse particles separated by the separator 20 are supplied onto the mill table 12.
The above-described fixed separator 20 is a cyclone separator, and has a simple structure without a driver. Thus, the fixed separator 20 has advantages such as low cost and easy maintenance. However, the fixed separator 20 exhibits a poor accuracy in coarse particle size separation, and coarse particles (coarse particles having a size of over 100 mesh and providing an adverse effect to combustibility) contained in pulverized coal increases. This causes an increase in unburnt combustibles contained in combustion exhaust gas discharged from the boiler.
The principle of separation by the fixed separator 20 will be briefly described. Particles of pulverized coal in the solid-gas two-phase flow having passed between adjacent ones of the fixed blades 23 through the fixed blade inlet window 22 are centrifugally separated into coarse particles and fine powder by a swirl flow. Subsequently, the lightweight fine powder having a small particle size rides on and rises with an inverted upward flow from below. Then, the fine powder enters the inner cylinder 24 from below, and flows out from the vertical roller mill 10 through the pulverized coal outlets 16. However, the centrifugalized coarse particles having a large particle size cannot ride, due to a high weight thereof, on the flow entering the inner cylinder 24 from below. For this reason, such particles reach an inner wall of the cone 21, and then, drop down along an inner wall surface of the cone 21 due to the force of gravity.
These coarse particles eventually drop down onto the mill table 12 through the coal feed tube 14 opening at a lower center portion of the cone 21, and are pulverized again.
In order to reduce the proportion of coarse particles in product pulverized coal in the above-described vertical roller mill including the fixed separator, the following conventional techniques are disclosed in Patent Literature 1: drift plates are provided in the vicinity of vane input windows; and inclination of an inner cylinder is changed.